Evidence that the feeder effect in mammalian cells is mediated by a diffusible substance

Chinese hamster ovary (CHO), baby hamster kidney (BHK), and NG108-15 neuroblastoma cells were heated and then plated for the colony-formation assay either with or without feeder cells present. Besides these cell lines, four other cell types were used as feeders. All cell lines functioned equally well as feeders for each of the heated cell lines. In some experiments the heated and feeder cells were separated by semipermeable membranes. This separation had no effect on the feeder effect, indicating that cell-to-cell contact was not requisite. The feeder effect appears to be mediated by a low molecular weight, diffusible substance produced by the feeder cells.